An agonist muscle is the source of the force needed to finish a movement and to achieve this it must contract (shorten) or relax (lengthen). As it is the agonist that produces the force, it is also referred to as the prime mover. An antagonist muscle works in an opposite way to the agonist. That is why these muscles must work in pairs.
Which of the following agonist and antagonist muscles are correctly paired? The hamstrings are the agonist and the quadriceps are the antagonist. In the contact and recovery phase, the quadriceps contract to extend the knee while the hamstrings lengthen to allow the movement. The quadriceps are the agonist and the hamstrings are now the antagonist.